The present invention relates to an image reducing system for reducing an image expressed by binary values respectively representing a white point and a black point. More particularly, the present invention relates to an image reducing system and method for converting a character image into a corresponding reduced character image without entailing significant deterioration of image quality.
Methods for reducing an image have been proposed such as the selective processing conversion method (SPC method), the logical OR method, the nine-division method and the fast projection method. Every one of those known methods determines the density of converted pixels on the basis of four adjacent original pixels. Henceforth, the term "converted pixel" means a pixel on a reduced image, and the term, nonconverted pixel means a pixel on an original image. The SPC method selects one original pixel nearest to a converted pixel and uses color information about the original pixel as the value of the converted pixel. The logical OR method determines the density (color information) of a converted pixel on the basis of the logical OR between the densities of four original pixels nearest to the converted pixel. The nine-division method divides a square block defined by four original pixels into nine partial blocks and determines the density of the converted pixel by using logical expressions predetermined for the nine partial blocks including convert ed pixels. The SPC, logical OR and nine-division methods are described in "Evaluation of Same Binary Image Enlargement and Shrink Methods and Stepped-Up Method" by H. Shojima, et al., Information Processing Society of Japan, September 1985, pp. 920-925. The fast projection method is describe in Japanese Patent Laid-open (Kokai) No. 58-97958 and "Fast Pixels Density Conversion Method Based on Projection Method", Institute of Image Electronics Engineers of Japan, Vol. 11, No. 2, pp.72-83 (1982).
Since the above-described known image reducing methods determine the density of the converted pixel on the basis of the four nonconverted pixels around the converted pixel, no reference is made to some of the nonconverted pixels in reducing the original image when the horizontal reduction ratio nx and the vertical reduction ratio ny are smaller than 1/2. For example, when the original image is reduced by the SPC method that selects the value of an original pixel nearest to a converted pixel, a fine line having a line width corresponding to m pixels disappears if any converted pixel is not included in the fine line. The rest of the image reducing methods also have the problem that a fine line disappears when the fine line is formed of pixels to which no reference is made.
An image reducing method capable of retaining fine lines, described in Japanese Patent Laid-open (Kokai) Nos. 1-238275 and 2-290369, and "Extended Thin Line Preservation Methods for Binary Image Reduction" by K. Wakabayashi, et al. Denshi Tsushin Gakkai Ronbun-shi, D-II, Vol. J75-D-II, No. 8, pp. 1364-1371 makes reference to a reference pixel block defined by the product 4(1/n.sub.x).times.(1/n.sub.y) of 2.times.(1/n.sub.x) horizontal rows of nonconverted pixels and 2(1/n.sub.y) vertical rows of nonconverted pixels, where n.sub.x is a horizontal reduction ratio and ny is a vertical reduction ratio when the horizontal reduction ratio n.sub.x and the vertical reduction ratio n.sub.y are smaller than 1/2 when calculating the density of the converted pixel. Henceforth, the term "original image reference pixel" means original pixel to which reference is made to retain a black line, and the term "reference pixel block" means a block including pixels to which reference is made in each reference cycle. Those methods match a fine line included in the reference pixel block to a predetermined pattern, determine that horizontal lines, vertical lines and oblique lines having a length greater than a predetermined length are fine lines, retains the fine lines and determines converted pixels.
When an original image is reduced by this known method, there is no converted pixel to which reference is not made in reducing an image even if the horizontal reduction ratio n.sub.x and the vertical reduction ratio n.sub.y are smaller than 1/2 and any fine line will not disappear. However, this known method has a problem that the line width of a line in a reduced image varies depending on the relation between the position of a white or black line in the original image and the position of the pixels of the reduced image.
This problem in the known method will be described with reference to FIGS. 20A and 20B. In FIGS. 20A and 20B, pixels indicated at S.sub.pq are reference pixels in an original image, and those indicated at R.sub.rs are reduced pixels. The horizontal reduction ratio n.sub.x =1/3 and the vertical reduction ratio n.sub.y =1/3. In FIG. 20A, pixels S.sub.01 to S.sub.04, S.sub.11 to S.sub.14, . . . and S.sub.71 to S.sub.74 represented by solid circles form a vertical black line having a line width d.sub.x =4-pixel width, i.e., a width corresponding to four pixels. Similarly, in FIG. 20B, pixels S.sub.02 to S.sub.05, S.sub.12 to S.sub.15, . . . and S.sub.71 to S.sub.75 represented by solid circles form a longitudinal line having a line width d.sub.x =4-pixel width, i.e., a width corresponding to four pixels.
The aforesaid known method defines fine black lines with a horizontal line width d.sub.x &lt;1/n.sub.x or a vertical line width d.sub.y &lt;1/n.sub.y. Therefore, in the example shown in FIGS. 20A and 20B, the black line cannot be detected because the line width d.sub.x =4-pixel width and 1/n.sub.x =3. In such a case, the known method, such as the SPC method or a fast projection method, calculates the values of nonconverted pixels in the neighborhood of the converted pixel.
Accordingly, when the black line in the original image lies at the position shown in FIG. 20A, the SPC method or the fast projection method maps the black line shown in FIG. 20A to reduced pixels R.sub.01 to R.sub.21 represented by solid squares and forming a black line having a line width corresponding to one pixel. For example, the SPC method selects the nearest original image pixel for mapping. Therefore, the pixel S.sub.02 is selected and mapped to the reduced pixel R.sub.01 and the reduced pixel R.sub.01 is a black pixel. Similarly, the pixel S.sub.32 is selected and mapped to the reduced pixel R.sub.11 and the pixel R.sub.11 is a black pixel, the pixel S.sub.35 is selected and mapped to the reduced pixel R.sub.12 and the reduced pixel R.sub.12 is a white pixel. The fast projection method determines the color of the reduced pixel on the basis of the colors of four pixels in the neighborhood of the reduced pixel, and the positional relation between those four pixels and the reduced pixel.
For example, since all four pixels S.sub.02, S.sub.12, S.sub.03 and S.sub.13 surrounding the reduced pixel R.sub.01 are black pixels, the reduced pixel R.sub.01 is a black pixel. Similarly, since all four pixels S.sub.32, S.sub.42, S.sub.33 and S.sub.43 surrounding the reduced pixel R.sub.11 are black pixels, the reduced pixel R.sub.11 is a black pixel, since all four pixels S.sub.05, S.sub.15, S.sub.06 and S.sub.16 surrounding the reduced pixel R.sub.02 are white pixels, the reduced pixel R.sub.02 is a white pixel, and since all four pixels S.sub.35, S.sub.45, S.sub.36 and S.sub.46 surrounding the reduced pixel R.sub.12 are white pixels, the reduced pixel R.sub.12 is a white pixel.
However, when the black line in the original image lies at the position shown in FIG. 20B, the SPC method or the fast projection method maps the black line of FIG. 20B to reduced pixels R.sub.01 to R.sub.21 and R.sub.02 to R.sub.22 represented by solid squares, so that a black line having a 2-pixel width is formed. For example, the SPC method selects the nearest original image pixel for mapping. Therefore, the original pixel S.sub.02 is selected and mapped to the reduced pixel R.sub.01 and hence the reduced pixel R.sub.01 is a black pixel; the original pixel S.sub.32 is mapped to the reduced pixel R.sub.11 and hence the reduced pixel R.sub.11 is a black pixel; the original pixel S.sub.05 is mapped to the reduced pixel R.sub.02 and hence the reduced pixel R.sub.02 is a black pixel; and the original pixel S.sub.35 is mapped to the reduced pixel R.sub.12 and hence the reduced pixel R.sub.12 is a black pixel. When the fast projection method is used, the reduced pixel R.sub.01 is a black pixel because all the four original pixels S.sub.02, S.sub.12, S.sub.03 and S.sub.13 surrounding the reduced pixel R.sub.01 are black pixels; the reduced pixel R.sub.11 is a black pixel because all the four original pixels S.sub.32, S.sub.42, S.sub.33 and S.sub.43 surrounding the reduced pixel R.sub.11 are black pixels; the reduced pixel R.sub.02 is a black pixel because two original pixels S.sub.05 and S.sub.15 nearest the reduced pixel R.sub.02 are black pixels; and the reduced pixel R.sub.12 is a black pixel because two pixels S.sub.35 and S.sub.45 nearest the reduced pixel R.sub.12 are black pixels.
Thus, in the examples shown in FIGS. 20A and 20B, when the original image is reduced in a reduction ratio of 1/3, the black line having a 4-pixel width in the original image is mapped to either a black line having a 1-pixel width or a black line having 2-pixel width, depending on the position of the line in the original image and the positions of the reduced pixels. Generally, when the original image is reduced by the foregoing known methods, the width of the black lines and the white lines in the reduced image formed by reducing the black line and the white line of the original image is not dependent uniquely on the position of the line in the original image and the positions of the reduced pixels, causing the distortion of the reduced image, i.e., the deterioration of the image quality.
Furthermore, reference must be made to 4(1/n.sub.x).times.(1/n.sub.y) pixels in the reference pixel block in the original image to reduce the original image in a horizontal reduction ratio n.sub.x and a vertical reduction ratio n.sub.y (n.sub.x and n.sub.y are smaller than 1/2) and, consequently, reference must be made four times or above to the original image.